Brief Overview
We work on a variety of topics in or related to X-ray astronomy, including galaxy clusters, AGN, surveys, X-ray binaries, and dark matter (or lack thereof). We want to learn:
- How do the largest structures form?
- When galaxies form, how is their structure and evolution regulated?
- What is dark matter and dark energy?
Galaxy Clusters
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| Understanding the hot gas in the largest objects in the universe: X-ray images of the galaxy cluster CL0217+17 from Tumer et al. (2023). |
Galaxy clusters are made up of 100s to 1000s of galaxies, bound by their mutual gravity AND gravity from dark matter and the diffuse gas in between the galaxies. In fact, most of the baryons (normal everyday matter) are in the hot intervening gas, and most of the matter is made up of dark matter. The gas emits X-rays, which better trace the properties of the cluster, such as its mass, history, and dynamical state.
Most of the X-rays we detect are thermal, being emitted by hot gas that is close to equilibrium with the gravitational potential of the cluster. However, in some galaxy clusters that recently formed from two clusters colliding with each other, we see non-thermal radio synchrotron emission, which means there are also non-thermal X-rays in addition to the thermal ones - just fewer of them. Our hunt for these elusive photons continues, which if found can tell us the strength of the magnetic field in clusters and how much energy is injected into the non-thermal phase of the gas. During these mergers, huge shocks are driven into the gas, heating it up as the shock wave passes. The exact process by which the gas is heated is unclear, but the temperatures can be quite high, and a high energy telescope like NuSTAR is needed to accurately constrain them.
Estimates of the masses of galaxy clusters are complicated not only by these concerns, but by questions around the accuracy of the temperature measurements themselves. NuSTAR provides a new window on the long-standing calibration discrepancies between X-ray telescopes, and we are investigating this issue in the hope that cosmological constraints using clusters can be improved.
X-ray Binaries in Nearby Galaxies
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| Understanding the neutron star and black hole populations of galaxies: Deep NuSTAR observations of the X-ray binary population of the Andromeda Galaxy (M31), which can differentiate between neutron star and black hole systems. |
Most stars are in binary systems, and in some of them - after one of the pair explodes, creating a neutron star or black hole - the newly dead star will feast on the still-burning companion, producing X-rays in the process. This vampiric (or zombie-like? which trope is more popular right now?!?) action can help identify the nature of the feeding beast - at least in our own Galaxy where we can detect the high energy X-rays that are crucial for distinguishing black holes from neutron stars. Unfortunately, it's much harder to do this outside of the Milky Way.
The launch of NuSTAR, however, changed all that. Its superior sensitivity at high energies makes it possible to perform this test in nearby galaxies for the first time. The NuSTAR survey of part of M31 is shown in the inset above, where the X-ray color strongly hints at the nature of the compact object (i.e., vampire/zombie). We are now expanding the NuSTARcoverage of M31 to better characterize its population as a whole.
The Cosmic X-ray Background
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| The total X-ray spectrum of the universe: Feeding supermassive black holes in the centers of galaxies thoughout the universe dominate the light of the high energy X-ray sky, and accurately measuring this emission constrains models of the growth of these black holes, and how they affect their host galaxies, over all of cosmic time. |
Since the birth of X-ray astronomy over 40 years ago, observatories have measured the cosmic X-ray background as a function of energy, but many of those measurements are in mild disagreement. It turns out to be notoriously difficult to absolutely calibrate these telescopes, which is largely due to the lack of a "standard candle" in the X-ray sky - an object whose brightness never changes - and the challenge of characterizing a telescope that undergoes the stresses of a rocket launch and then operates in microgravity without humans being able to tinker or even look at it again after launch. The amount of radiation in the X-ray background is important because it contains the entire history of accretion onto supermassive black holes, and differences of 10% in the normalization or shape of the background spectrum can translate into vastly different evolutionary histories of the growth of AGN.
Through a quirk of design, the NuSTAR Observatory - sensitive at exactly the energies where these discrepancies are greatest - routinely detects the cosmic X-ray background as "stray light" shining directly on the detectors. We have combined the archive of NuSTAR observations to precisely measure how bright and what color the X-ray background is, and what that implies about the types of AGN in the universe. Our measurements fall below most others around 10 keV, as can be seen above, but agree quite well with measurements from the only experiment specifically designed to do this: HEAO-1. This discrepancy is likely related to the calibration differences that result in discrepant galaxy cluster temperatures (see above).
X-ray AGN Surveys
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| Finding individual supermassive black holes near and far: These feeding black holes, or active galactic nuclei (AGN), grow sporadically and in different ways over time, which can be revealed in surveys such as in the North Ecliptic Pole (NEP) field above. |
Surveys at all wavelengths are one of the most fundamental ways we learn about objects in the universe, and in the X-ray they are typically dominated by AGN. These black holes may be partaking of a light snack or a ginormous feast, and they may be across the universe or practically right next door. Taking a census of these objects and their activity provide key insights about how and why they grow. In the smaller galaxies, these black holes can be particularly hard to find, so targeted surveys of nearby galaxies are needed. Students in the group take part in these kinds of investigations.
Future X-ray Observatories
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| Continuing the Expedition: Much remains to be discovered, and the only way to make progress is to build new, even more capable telescopes to answer the questions left unanswered. For example, galaxy clusters may contain clumps (above, from Norseth et al. 2023), that we cannot currently detect; a new mission like STAR-X is needed to find out. |
New missions are crucial for the continued advancement of science. We here love all potential new X-ray observatories and get excited about using them right out of the box when possible. The latest observatory, XRISM, launched in September of 2023 and is providing high resolution, non-dispersive spectra of (especially) diffuse objects - like galaxy clusters - for the first time. We also participate in proposals for new missions, like the MIDEX class observatory STAR-X and the Probe class observatory HEX-P.
